1. Field of the Invention
The present invention relates to an image forming apparatus according to an electrophotographic system having a development device.
2. Description of the Related Art
Generally, in an image forming apparatus according to an electrophotographic system, an image is formed by an image forming process. The image forming process includes a charging process step, an exposure process step, a development process step, a transfer process step, a fixing process step, and a cleaning process step.
In forming images, first, the surface of an electrophotographic photosensitive member (in the following, referred to as a photosensitive member) is uniformly charged. After that, the photosensitive member is exposed according to image information to form an electrostatic latent image. Subsequently, a toner is supplied to the electrostatic latent image, and then the electrostatic latent image is turned into a toner image. The toner image formed on the photosensitive member is then transferred on a recording material such as a paper sheet. The recording material on which the toner image is transferred is heated and pressurized in the fixing process step. Thus, the toner image is fixed on the surface of the recording material. Therefore, forming the image on the recording material is ended. On the other hand, the toner remains on the surface of the photosensitive member after the toner image is transferred. The residual transfer toner is removed in the cleaning process step.
Heretofore, particularly in an image forming apparatus that forms images in chromatic colors among image forming apparatuses according to the electrophotographic system, a two component development system is widely used, in which a nonmagnetic toner (a toner) and a magnetic carrier (a carrier) are mixed and used for a developer. As compared with other presently known development systems, the two component development system has advantages such as the stability of image quality and the durability of the apparatus.
In the image forming apparatus using the two component development system, in the case where an electrostatic latent image formed on a photosensitive drum that is an image bearing member is developed to form a toner image, generally, the image is formed as follows. The surface of the photosensitive drum is first uniformly charged by a charging portion in such a way that the potential of the photosensitive drum is at a white base portion potential Vd. Moreover, a developing bias voltage is applied to a developing sleeve that is a developer bearing member, and the potential of the developing sleeve is made to the same as a direct current component Vdc of the developing bias voltage.
In this application, a potential difference between the white base portion potential Vd and the direct current component Vdc of the developing bias voltage is set to a desired fog removal potential difference Vback. Moreover, an image portion (a developed portion) on the photosensitive drum is exposed by an exposing portion that forms an electrostatic latent image, and the potential of the image portion is made at an attenuated bright portion potential VL. Then, a contrast potential difference Vcont that is a difference between the bright portion potential VL and the direct current component Vdc of the developing bias voltage causes a toner on the developing sleeve to move to the photosensitive drum. As described above, the electrostatic latent image formed on the photosensitive drum is developed as a toner image.
Generally, in such a two component development system, the toner is consumed by forming images, and the toner is replenished accordingly. Thus, forming images is repeated to sequentially replace the toner in the developer.
However, particularly in the image forming apparatus using the two component development system, the following problem arises.
In these years, various types of paper sheets are used for recording materials. For the recording materials, there are many types of recording materials such as inexpensive paper sheets with a low surface smoothness (mainly recycled paper sheets) and paper sheets with a high smoothness such as coated paper sheets. Among these recording materials, particularly in the case of a paper sheet with a low smoothness, the transfer characteristic of a toner is altered along the surface shape of the paper sheet, and transfer irregularity tends to occur.
Generally, a transfer condition (a transfer bias) to a paper sheet is changed according to the setting of a paper sheet type (a plain paper sheet, recycled paper sheet, thick paper sheet, OHT, and so on) selected by a user. However, in the case where a toner is degraded due to long-term use and a temperature rise, it is difficult to keep the transfer characteristic to a paper sheet through long-term use.
The following are situations that a toner tends to be degraded. In the case where images (images with a low image ratio) that use a low amount of a toner are continuously output, the toner is rarely replaced. In this case, time for which a single toner exists in the developing device is prolonged, and the toner is kept circulating for a long time in the developing device.
When the dwell time of the toner is prolonged in the developing device, the toner is repeatedly rubbed and stirred in the developing device for a long time, causing the case where the toner shape becomes irregular and the case where the grain size distribution is unbalanced. Moreover, an additive, which is added to a developer to improve flowability, is sometimes embedded in the toner surface. As a result of these problems, when degradation occurs such as a decrease in the flowability of the developer, an image of a desired image quality might not be obtained.
For a method for solving these problems, in Japanese Patent Laid-Open No. 2006-337699, a unit that calculates the image ratio of an image being formed is provided, in which when it is detected that the calculated image ratio is below a predetermined value, a predetermined amount of a toner is developed in a non-image region to forcedly consume the toner. The amount of a toner consumed is then newly replenished in a developing device. Thus, the degraded toner is replaced by a new toner.
This control is performed to prevent the toner in the developing device from not being replaced in the case where images with a low image ratio are continuously kept output. Thus, reductions in image quality and image density are prevented. Here, particularly in the case of using a paper sheet with a low smoothness such as a recycled paper sheet, it is necessary to replace a larger amount of a toner in order to maintain the transfer characteristic to the paper sheet as compared with the case of using a paper sheet with a high smoothness.
The situations that a toner tends to be degraded will be further described. With the downsizing of an electrophotographic apparatus in these years, the device internal temperature of an image forming apparatus main body tends to rise caused by heat generated from a fuser and an electric circuit board. Thus, in association with the temperature rise in the main body, it is likely to soften a toner in the development device. It is noted that generally, the softening point of a toner is temperatures of about 60 to 80° C.
Moreover, in the case where the user turns off a power supply, the rotation of a cooling fan that cools the inside of the main body is also stopped. The temperature of the development device where the cooling performance is reduced then suddenly rises, and sometimes partially exceeds the softening point of a toner. The toner is softened and degraded, sometimes causing image quality to be considerably reduced under a high temperature environment.
For a method for solving these problems, in Japanese Patent Laid-Open No. 5-257358, even in the case where the power supply of an image forming apparatus main body is turned off, a cooling fan is operated until the temperature of a development device reaches a preset temperature. Such control is performed to prevent a sudden temperature rise in the development device in the case where the power supply of the main body is turned off, and to suppress the degradation of a toner caused by softening the toner in the development device. Thus, the degradation of image quality can be prevented.
Here, particularly in the case of using a paper sheet with a low smoothness such as a recycled paper sheet, it is necessary to reduce the temperature of the development device (the temperature of a toner) as compared with the case of using a paper sheet with a high smoothness in order to maintain the transfer characteristic to a paper sheet.
However, in the case of Japanese Patent Laid-Open No. 2006-337699, a toner replacement amount is constant regardless of paper sheet types. Thus, in the case where the toner replacement amount is set larger as matched with a paper sheet with a low smoothness, a toner is wasted when using a paper sheet with a high smoothness. On the contrary, when the toner replacement amount is set smaller as matched with a paper sheet with a high smoothness, it is difficult to maintain the transfer characteristic of a toner to a paper sheet in the case of using a paper sheet with a low smoothness.
Moreover, in the case of Japanese Patent Laid-Open No. 5-257358, the temperature threshold of the development device at which it is determined whether the cooling fan is operated is constant regardless of paper sheet types. Thus, when the temperature at which the cooling fan is operated is set lower as matched with a paper sheet with a low smoothness, the fan is excessively operated in the case of using a paper sheet with a high smoothness. This excessive operation causes a harmful effect such as a reduction in the lifetime of a fan motor and noise caused by operating the fan. On the contrary, when the temperature at which the cooling fan is operated is set higher as matched with a paper sheet with a high smoothness, it is difficult to maintain the transfer characteristic of a toner to a paper sheet in the case of using a paper sheet with a low smoothness.